Changes in water microbial quality during bank filtration of lake water

Abstract

We studied, during a 2-year period, how the microbial stability of water changed when humus-rich lake water was filtered through the ground at a bank filtration water plant. The changes in microbial quality were followed as microbial numbers and growth activity. The filtration decreased microbial counts and growth ([³H]thymidine technique) in water up to 90%. The reduction in bacterial counts and growth depended on the filtration distance. The reduction was greatest between the lake and the first sampling point. Microbial numbers and growth declined steadily after infiltration with increased filtration distance. Viable counts of heterotrophic bacteria decreased faster than total bacterial counts along filtration. The microbial numbers and bacterial production in water followed seasonal changes in water temperature. Simultaneously with the microbial numbers, the concentrations of total organic carbon and assimilable organic carbon decreased during bank filtration. These results showed that microbial stability of humus-rich water was increased by filtration to a level generally found in natural groundwaters.Key words: bacteria, bank filtration, biological stability, drinking water, groundwater.